1. Field of the Invention
The present invention relates to a packet communication system and a method of assigning timeslots of the system. The present invention is particularly concerned with a dynamic timeslot assignment method for a common medium point-to-multipoint communication system in which multiple local units are connected to a network unit via a common transmission medium and the network unit dynamically assigns timeslots to the local units under varying traffic.
2. Description of the Related Art
A prior art point-to-multipoint communication system is comprised of a plurality of line concentrators on the user side of the system and a timeslot assignment unit on the network side. Each line concentrator multiplexes (i.e., concentrates) traffic from a plurality of user terminals onto an optical local access line which is coupled by an optical coupler to a common optical line. Each line concentrator includes a buffer for temporarily storing ATM cells received from the associated user terminals and a queue length detector is provided to count the number of outstanding cells in the buffer that form a queue waiting for transmission and sends a queue length signal to the timeslot assignment unit. In order to provide efficient utilization of the common medium for bursty traffic, the timeslot assignment unit dynamically assigns timeslots to the line concentrators based on the queue lengths of the line concentrators, as disclosed in Japanese Patent Publication 10-242981. According to the prior art timeslot assignment technique, timeslots are assigned to each line concentrator at periodic intervals S as shown in FIG. 1, where the interval S equals to or an integral multiple of the length of a frame. At each assignment time, the timeslot assignment unit determines the number of timeslots to be assigned to each line concentrator during each interval S, and then determines the number of timeslots to be assigned during each frame if the assignment interval S is an integral multiple of the frame length and their position within a frame to produce a slot position signal, which is sent to each line concentrator.
However, there is an inherent control delay in this timeslot assignment process. Specifically, this control delay is a total sum of the intervals for timeslot assignment calculations, the propagation delay time, and the time to establish synchronization with system clock at each side of the network.
As shown in FIG. 1, at time t0, each line concentrator communicates a queue length signal Q0=100 to the timeslot assignment unit 13, indicating that there are one-hundred ATM cells forming a queue in the buffer 6. As long as they remain in the buffer, queue length signals Q0 will be transmitted at update intervals. Based on a received queue length signal, the timeslot assignment unit 13 calculates the count number G0 (=40, for example) of timeslots to be assigned during an assignment period S0. If the update interval S is equal to the length of a frame, the assignment unit 13 determines the slot positions of assigned timeslots in a frame at time t1−α and sends a signal g0-i to the associated line concentrator for indicating the frame-by-frame timeslot count number and the timeslot position (where i indicates frame number). Timeslot assignment unit 13 successively calculates the numbers of timeslots G1=50 and G2=60 at times t2−α and t3−α in response to queue length signals Q1=100 and Q2=100 and produces timeslot identification signals g1-i and g2-i.
It is seen that the value G0=40 produced at time t0 is actually used by the line concentrator at time t3 that is delayed by a period of 3S with respect to time t0. In the same way, the assigned timeslot count numbers G1=50 and G2=60 produced at times t1 and t2 are actually used by the concentrator at times t4 and t5. The presence of such control delay implies that there are cells in the buffer 6 which were already assigned timeslots but are still waiting for their turn to be forwarded to the network. For example, at time t3, there are 100 outstanding cells in the buffer that were already assigned timeslots whose total number equals 150 (=40+50+60).
However, because of the delay time fifty timeslots are unnecessarily assigned to the one-hundred outstanding cells. Since the surplus timeslots are not used by the line concentrators, they result in a low throughput in cell transfer. In addition, the delay time associated with the outstanding cells in the buffer and hence the buffer capacity for a given cell loss rate is proportional to the length of the queue. Due to the low cell transfer throughput, there is an increase in cell delay, hence an increase in required buffer capacity.